1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine, and more particularly to a fuel supply apparatus for an internal combustion engine supplying the internal combustion engine with a fuel by using low-pressure fuel supply means and high-pressure fuel supply means.
2. Description of the Background Art
Examples of a method of supplying a fuel to an internal combustion engine such as a gasoline engine and a diesel engine incorporated in a vehicle such as a passenger car, a truck, or the like include in-cylinder injection for directly injecting the fuel into a combustion chamber within a cylinder of the internal combustion engine, port injection for injecting the fuel into an intake port for taking air into the combustion chamber within the cylinder of the internal combustion engine, and a combination thereof, that is, in-cylinder injection/port injection switching between in-cylinder injection and port injection in accordance with an operation state of the internal combustion engine.
For example, Japanese Patent Laying-Open No. 7-103048 discloses a fuel supply apparatus for an internal combustion engine carrying out in-cylinder injection/port injection in accordance with an operation state of the internal combustion engine described above. The fuel supply apparatus for the internal combustion engine (a fuel injection apparatus) includes a port fuel injection valve serving as low-pressure fuel supply means for performing port injection (a fuel injection valve for injection into an engine intake manifold) and an in-cylinder fuel injection valve serving as high-pressure fuel supply means for performing in-cylinder injection (a fuel injection valve for in-cylinder injection). The fuel supply apparatus for the internal combustion engine controls injection from the in-cylinder fuel injection valve and the port fuel injection valve, that is, controls injection timing and a quantity of injection, in accordance with a map prepared based on a fuel supply quantity (a fuel injection quantity) and degree of accelerator press-down (an amount of press-down of an accelerator pedal). Specifically, the map is divided into an injection region where solely the in-cylinder fuel injection valve is used, an injection region where both of the in-cylinder fuel injection valve and the port fuel injection valve are used, and an injection region where solely the port fuel injection valve is used. Then, an ECU (Engine Control Unit) controls injection from the in-cylinder fuel injection valve and/or the port fuel injection valve in accordance with the operation state of the internal combustion engine.
Generally, when the fuel is injected into the combustion chamber within the cylinder from the in-cylinder fuel injection valve as described above, the fuel supply apparatus for the internal combustion engine should inject a high-pressure fuel. Therefore, the fuel supply apparatus includes a high-pressure pump for supplying a high-pressure fuel to the in-cylinder fuel injection valve. The high-pressure pump applies pressure to the fuel in the following manner. A cam for the pump attached to an intake camshaft or an exhaust camshaft rotates as a result of transfer of rotation force from a crankshaft of the internal combustion engine, and a plunger is caused to carry out reciprocating motion. Then, the fuel to which pressure has been applied by a low-pressure pump is suctioned into a pressurizing chamber of the high-pressure pump, in which further pressure is applied.
The high-pressure pump continues to be driven by rotation of the crankshaft of the internal combustion engine, even under the control by an ECU so as not to supply the fuel from the high-pressure fuel supply means to the internal combustion engine, that is, so as not to inject the fuel from the in-cylinder fuel injection valve. Consequently, pulsation is produced when the high-pressure pump suctions the fuel from a high-pressure fuel system or when excessive fuel is returned. Pulsation fluctuates pressure of the fuel, i.e., fuel pressure, in the high-pressure fuel system and the low-pressure fuel system. Such fluctuation in the fuel pressure propagates to the low-pressure fuel supply means, that is, a low-pressure fuel delivery pipe supplying the fuel within a low-pressure pipe to the port fuel injection valve provided corresponding to each cylinder or a fuel injection valve for each port. Though the ECU controls injection timing and a quantity of fuel to be injected from the port fuel injection valve to the intake port in an intake system of the internal combustion engine in accordance with the operation state of the internal combustion engine, the port fuel injection valve has not been able to inject the fuel of an injection supply quantity, that is, a fuel injection quantity, determined based on the operation state of the internal combustion engine, due to propagation of pulsation to the low-pressure fuel delivery pipe or to the port injection valve.
Particularly in a V-type 6-cylinder engine including two cylinder groups each consisting of three cylinders, pulsation generated from the high-pressure pump propagates to the low-pressure fuel delivery pipe provided in each cylinder group, and further to the port fuel injection valve provided for each cylinder from the low-pressure fuel delivery pipe. Here, if a pipe from the high-pressure pump to each low-pressure fuel delivery pipe has the same length, a phase of a cycle of a magnitude of pressure fluctuation caused by pulsation, of the fuel supplied to the port fuel injection valve is the same. If injection timing of each port fuel injection valve has a cycle half the magnitude of pressure fluctuation of the fuel, the fuel is injected from the port fuel injection valve provided in one cylinder group at the time when the magnitude of pressure fluctuation of the fuel attains an upper limit, whereas the fuel is injected from the port fuel injection valve provided in the other cylinder group at the time when the magnitude of pressure fluctuation of the fuel attains a lower limit. That is, if a valve-open time period, i.e., an electrified time period, of the port fuel injection valves is the same among one another under the control of the ECU, variation in a quantity of fuel injected from the port fuel injection valve for each cylinder group becomes significant. As described above, the fuel supply apparatus has not been able to supply the fuel of a quantity to be supplied to the internal combustion engine, and an air-fuel ratio representing a ratio between air and the fuel has disadvantageously fluctuated.